Experimental and Analytical Investigation of a Two-Phase Ejector Used for Expansion Work Recovery in a Transcritical R744 Air-Conditioning System
Elbel, Stefan
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https://hdl.handle.net/2142/83897
Description
Title
Experimental and Analytical Investigation of a Two-Phase Ejector Used for Expansion Work Recovery in a Transcritical R744 Air-Conditioning System
Author(s)
Elbel, Stefan
Issue Date
2007
Doctoral Committee Chair(s)
Hrnjak, Predrag S.
Department of Study
Mechanical Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Language
eng
Abstract
Although the reduced cooling capacity and coefficient of performance (COP) at elevated outdoor temperatures represent an inherent drawback of the transcritical carbon dioxide (CO2, R744) refrigeration cycle, the supercritical heat rejection pressure can be used to maximize the system performance. In addition, large throttling losses make R744 a suitable fluid for expansion work recovery. This study presents experimental and computational results obtained from a transcritical R744 air-conditioning system using a two-phase ejector specifically designed and built for this investigation. The results are compared with that of a conventional system with an expansion valve. For the test conditions considered, the cooling capacity and COP simultaneously improved by up to 8% and 7%, respectively. Extrapolation was used to determine that the COP could have been improved by as much as 18% at matched cooling capacities. An ejector efficiency based on simple external pressure, temperature, and mass flow rate measurements was defined. According to this new metric, up to 17% of the expansion work potential was recovered. The ejector system COP was successfully maximized by using an integrated high-side pressure control mechanism. A control strategy ensuring ejector system operation with maximum energy efficiency was derived in accordance with the trends successfully predicted by numerical simulation results. Furthermore, it was shown that high outdoor temperatures and small diffuser angles both had positive effects on ejector efficiency. The interference between the ejector and an internal heat exchanger (IHX) was also investigated. It was found that the ejector system could have a reduced IHX effectiveness and still achieve capacities and COPs that were comparable to those of the conventional expansion valve system with IHX. However, maximum performance was obtained with a highly effective IHX and ejector. High-speed flow visualization was used to investigate the complex two-phase shock wave patterns observed in the mixing section of the ejector. Static wall pressure distributions along the ejector were analyzed and flow choking phenomena and metastabilty effects were studied. Finally, a lighter weight and significantly smaller second generation ejector displayed even higher ejector efficiencies, up to 22%.
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